1. Field of the Invention
This invention relates to an image forming apparatus for forming images on recording media using an electrophotographic method.
2. Description of Related Art
Image forming apparatuses such as multicolor electrophotographic printers of an intermediate transfer method, generally have plural image forming units including such as, e.g., photosensitive drums, charge units, exposure units, and development units. The image forming units are provided sequentially along a medium conveyance route; a toner image is primarily transferred to an intermediate transfer belt from the image forming units of respective colors; and the toner image primarily transferred is secondarily transferred to the recording medium.
With such an image forming apparatus, the secondary transfer voltage is determined on the basis of electrical characteristics of the secondary transfer roller sought by measuring a secondary transfer current flowing when a prescribed secondary transfer voltage is applied where no recording medium exists at a and the opposed secondary transfer roller. The temperature inside the apparatus may change due to printing operation for a long time, and even where the electrical characteristic of the secondary transfer roller is changed, the electrical characteristics of the secondary transfer roller can be measured again by utilizing sheet interval (or namely printing interval) of the recording medium (see, Japanese Patent Application Publication No. 2010-2753 (A1), and Japanese Patent Application Publication No. 2014-106413 (A1)).
For example, where the recording medium is a continuous medium or medium in a web form, the recording medium always exists at the secondary transfer nipping portion, so that the electrical characteristics of the secondary transfer roller cannot be measured until the completion of printing. Where the printing operation is stopped as in a state that the recording medium exists at the secondary transfer nipping portion during the printing operation, the electrical characteristics of the secondary transfer roller cannot be measured at a time when resuming printing in substantially the same way. There arise problems that the secondary transfer voltage cannot be controlled in an optimum way, with prior art.
As an alternative method measuring the elected characteristics of the secondary transfer roller, a method can be thought in which the temperature value of the secondary transfer roller is measured to presume resistance change from the temperature value of the secondary transfer roller. With the temperature value measuring method using such as a contact type thermistor directly contacting the secondary transfer roller, because the surface of the secondary transfer roller is structured of a material including foamed urethane, the contact type or the secondary transfer roller may be worn according to rotation of the secondary transfer roller. To the contrary, a non-contact type thermistor is expensive by itself, and errors may occur in the measured temperature values due to attachment of toner. That is, many problems may occur in the method for presuming resistance change of the secondary transfer roller by directly measuring the temperature value of the secondary transfer roller.
It is therefore an object of the invention to provide an image forming apparatus capable of optimally controlling a secondary transfer voltage even where the resistance value of the secondary transfer roller is changed during printing operation using a continuous medium or web.